1. Field of the Invention
The present invention relates to an apparatus and method for providing a Multicast and Broadcast Service (MBS) in a Broadband Wireless Access (BWA) system. More particularly, the present invention relates to an apparatus and method for generating an MBS packet that supports timing and packet synchronization.
2. Description of the Related Art
As known in the art, communication systems were originally developed to provide voice services. As technology has evolved, communication systems are now able to provide a data service and various multimedia services as well as the voice services. However, conventional communication systems have failed to satisfy the increasing users' demands for services because of a relatively low bandwidth and an expensive service fee. Moreover, due to the ever-increasing demand for Internet services, the necessity for communication systems capable of efficiently providing the Internet services is increasing. As a result of these trends, Broadband Wireless Access (BWA) systems have been introduced to provide efficient data, multimedia and Internet services.
The BWA systems support various speeds of a variety of data services and multimedia application services (e.g., high quality moving pictures) as well as voice services. The BWA systems can access a Public Switched Telephone Network (PSTN), a Public Switched Data Network (PSDN), an Internet network, an International Mobile Telecommunication-2000 (IMT-2000) network, an Asynchronous Transfer Mode (ATM) network and the like in a mobile or fixed environment based on a wireless medium using a broadband frequency (e.g., 2 GHz, 5 GHz, 26 GHz, 60 GHz, etc.). Also, the BWA systems can support a channel transfer rate of 2 Mbps or more. According to the mobility of a Mobile Station (MS) (i.e., whether it is moving or fixed), a communication environment (i.e., indoor or outdoor), and a channel transfer rate, the BWA systems can be classified into a broadband wireless subscriber network, a broadband mobile access network, and a high-speed wireless Local Area Network (LAN).
A wireless access method of the BWA system is standardized by the Institute of Electrical and Electronics Engineers (IEEE) 802.16 standardization group, which is an international standardization group.
According to the IEEE 802.16 standards, by using a wide data bandwidth, a greater amount of data can be transmitted in a shorter period of time compared to a conventional wireless technology for voice service. In addition, a channel (or a resource) can be shared by all users, resulting in more efficient use of the channel. Moreover, since a Quality of Service (OoS) is ensured, the users can receive different QoSs on the basis of service features.
IEEE 802.16 systems conform to a Multicast and Broadcast Service (MBS) standard that can provide multicast and broadcast services to a plurality of Mobile Stations (MSs). According to the MBS standard, the same MBS zone can be distinguished using a different Connection IDentifier (CID) or a different Security Association (SA). That is, an MBS zone (i.e., an MBS_ZONE) can be distinguished through a CID and an SA. A Base Station (BS) broadcasts MBS_ZONE information through a Downlink Channel Descriptor (DCD) message. That is, it can be said that the MBS_ZONE is a group of BSs using the same CID and SA.
An MBS is divided into a single-BS access and a multi-BS access according to a service access method of an MS. When using the single-BS access method, an MS receives an MBS from one BS in whose service coverage area the MS resides. When using the multi-BS access method, an MS simultaneously receives an MBS from two or more BSs. FIG. 1 illustrates the single-BS access method according to the conventional art. FIG. 2 illustrates the multi-BS access method according to the conventional art.
Referring to FIG. 2, in the multi-BS access method, when an MS is located in an overlapping service coverage area between a serving cell and a neighboring cell, a signal from the neighboring cell does not act as noise caused by interference but acts as a signal gain as a result of Radio Frequency (RF) combining. This is called a macro diversity effect. However, the macro diversity effect can be obtained only when the same signal is transmitted from a serving BS and a neighboring BS. Therefore, in order to provide an MBS, all BSs existing within an MBS_ZONE must transmit the same signal at the same time.
As such, when an MBS is provided, there is a need for timing synchronization such that all BSs existing within the same MBS_ZONE can transmit the same signal at the same time.
As known in the art, a BS fragmentizes or packs a Service Data Unit (SDU) (e.g., an Internet Protocol (IP) packet) received from a network (core service network) to generate a Media Access Control (MAC) Packet Data Unit (PDU). Thereafter, the BS generates a PHYsical (PHY) layer burst by aggregating in the generated MAC PDUs and encodes the PHY layer burst according to a Modulation and Coding Scheme (MCS) level. The reason why a BS fragmentizes and packs a packet received from a network as above is that the packet size is variable. In the case of a multi-BS MBS, it is desirable that an Access Service Network GateWay (ASN_GW, or an MBS controller) performs a fragmentation and packing operation for synchronization, and a BS bypasses a received packet without fragmentation and packing.
That is because, if BSs existing within the same MBS zone separately fragmentize or pack an MBS packet received from a network without considering synchronization, there is a problem that the BSs may generate different sized MAC PDUs for the same content, thus not being capable of transmitting a corresponding content at an exact synchronization time.
In other words, a macro diversity effect on an MBS can be obtained only when a BS avoids a fragmentation or packing operation for an SDU (e.g., an IP packet). In order for the BS to avoid the fragmentation or packing operation, a packet received from a network must be accepted within a MAC PDU. Therefore, a need exists for an apparatus and method for generating an MBS packet that supports timing and packet synchronization.